1. Field of the Invention
The present invention is related to a rotation speed detecting apparatus for a motor having an encoder, and particularly to a rotation speed detecting apparatus for a motor, which properly divides the frequency of the pulse signal generated from the encoder, thereby exactly detecting the rotation speed of the motor.
2. Description of the Prior Art
Most servo systems perform the desired control after detecting the rotation speed of the motor by an encoder attached to the motor.
In the conventional rotation speed detecting system for a motor, the system uses either a F/V convertor (frequency to voltage convertor) or the number of encoder pulses per a predetermined sampling time. However, the conventional rotation speed detecting systems have a problem in that when the motor rotates at a low speed, the detection errors becomes high.
In order to solve this problem, a system has been recently developed which detects the duration of each encoder pulse signal with a clock pulse with a high frequency used in a microprocessor. That is, the system detects the rotation speed of the motor by counting the number of clock pulses which occur between each successive rising or falling edge of the encoder pulse signal.
Accordingly, the greater the difference between the duration of each encoder pulse and the period of the clock pulse, the less the detection error will be.
However, when the motor is rotated at high speed, the difference between the duration of each encoder pulse and the period of the clock pulse becomes relatively small. Consequently, there is a problem in that the detection error in the rotation speed increases because the number of clock pulses which occur between each successive rising or falling edge of the encoder pulse signal is relatively reduced.
On the other hand, a typical servo control apparatus is disclosed in this Japanese Patent Laid-Open No. 123267 (1988). The apparatus disclosed in Japanese Patent Publication properly divides the frequency of a feedback pulse from a servo motor encoder by a frequency driving circuit, thereby providing enough time for the microprocessor to perform the required operation.
However, the invention disclosed in the Japanese Patent Publication has the limited purpose of executing the smooth scanner action to a wide variable power. Furthermore, the apparatus is limited in that the maximum division number N is small and it can only perform discontinuous division by the Nth power of 2, where N is a positive integer including zero.